Scholarly Works (4 results)

In this dissertation we study the properties of active galactic nuclei (AGN), which are powered by the accretion activity of supermassive black holes residing at the centers of galaxies. While observations propose that growth of AGN and galaxies are globally tied, we investigate whether this connection exists in individual galaxies. We also investigate various AGN selection techniques and star formation rate (SFR) estimates using multi-wavelength data from Chandra, Spitzer and rest-frame optical spectra from the Keck telescope.

We find that combining multi-wavelength identification techniques provides a more complete AGN sample, as each selection method suffers from selection biases. In particular, all selection techniques are biased against identifying AGN in lower mass galaxies. Once stellar mass selection biases are taken into account, we find that AGN reside in galaxies with similar physical properties (i.e., SFR) as inactive galaxies.

We find that while AGN are prevalent in both star-forming and quiescent galaxies, AGN of a given accretion rate are more likely to reside in star-forming galaxies. The probability of fueling an AGN does not strongly depend on SFR for a star-forming galaxy, though it decreases when star formation is shut down in quiescent galaxies. We also find no evidence for a strong correlation between SFR or stellar mass of the host galaxy and AGN luminosity. These results indicate that while both AGN and galaxy growth are reliant on the same fuel, enhanced star formation activity does not necessarily go hand-in-hand with increased AGN activity. While the star formation activity of galaxies can be traced with various indicators, our investigations indicate that extrapolations from mid-infrared data using calibrations based on local galaxies overestimates SFRs at higher redshift. We show that a combina- tion of mid-infrared and far-infrared data provide a more reliable SFR estimation than the mid-infrared data alone. We also find that the robustness of UV-based SFRs depends on the extinction correction method used. We find a relatively small fraction of z ∼ 2 galaxies have SFRs from infrared observations that are elevated relative to other SFR tracers, and we do not find any contribution from AGN in this excess.

Using data from the MOSFIRE Deep Evolution Field (MOSDEF) survey, we present
a census of AGN-driven ionized outflows in a sample of 159 AGNs at $1.4 \le z
\le 3.8$. The sample spans AGN bolometric luminosities of $10^{44-47}
\mathrm{~erg~s}^{-1}$ and includes both quiescent and star-forming galaxies
extending across three orders of magnitude in stellar mass. We identify and
characterize outflows from the \hbeta, [OIII], \halpha ~and [NII] emission line
spectra. We detect outflows in $17\%$ of the AGNs, seven times more often than
in a mass-matched sample of inactive galaxies in MOSDEF. The outflows are fast
and galaxy-wide, with velocities of $\sim 400-3500 ~\mathrm{km~s}^{-1}$ and
spatial extents of $0.3-11.0$ kpc. The incidence of outflows among AGNs is
independent of the stellar mass of the host galaxy, with outflows detected in
both star-forming and quiescent galaxies. This suggests that outflows exist
across different phases in galaxy evolution. We investigate relations between
outflow kinematic, spatial, and energetic properties and both AGN and host
galaxy properties. Our results show that AGN-driven outflows are widespread in
galaxies along the star-forming main sequence. The mass-loading factors of the
outflows are typically $0.1-1$ and increase with AGN luminosity, capable of
exceeding unity at $L_\mathrm{AGN} \gtrsim 10^{46.3} \mathrm{~erg~s}^{-1}$. In
these more luminous sources the ionized outflow alone is likely sufficient to
regulate star formation, and when combined with outflowing neutral and
molecular gas may be able to quench star formation in their host galaxies.

We present ionized gas kinematics for 708 galaxies at $z\sim 1.4-3.8$ from
the MOSFIRE Deep Evolution Field survey, measured using models which account
for random galaxy-slit misalignments together with structural parameters
derived from CANDELS Hubble Space Telescope (HST) imaging. Kinematics and sizes
are used to derive dynamical masses. Baryonic masses are estimated from stellar
masses and inferred gas masses from dust-corrected star formation rates (SFRs)
and the Kennicutt-Schmidt relation. We measure resolved rotation for 108
galaxies. For the remaining 600 galaxies we use models based on HST imaging
structural parameters together with integrated velocity dispersions and
baryonic masses to statistically constrain the median ratio of intrinsic
ordered-to-disordered motion, $V/\sigma_{V,0}$. We find $V/\sigma_{V,0}$
increases with increasing stellar mass and decreasing specific SFR (SSFR).
These trends may reflect marginal disk stability, where systems with higher gas
fractions have thicker disks. For galaxies with detected rotation we assess
trends between their kinematics and mass, SSFR, and baryon surface density
($\Sigma_{\mathrm{bar},e}$). Intrinsic dispersion correlates most with
$\Sigma_{\mathrm{bar},e}$ and velocity correlates most with mass. By comparing
dynamical and baryonic masses, we find that galaxies at $z\sim 1.4-3.8$ are
baryon dominated within their effective radii ($R_E$), with Mdyn/Mbaryon
increasing over time. The inferred baryon fractions within $R_E$,
$f_{\mathrm{bar}}$, decrease over time, even at fixed mass, size, or surface
density. At fixed redshift, $f_{\mathrm{bar}}$ does not appear to vary with
stellar mass, but increases with decreasing $R_E$ and increasing
$\Sigma_{\mathrm{bar},e}$. For galaxies at $z\geq2$, the median inferred baryon
fractions generally exceed 100%. We discuss possible explanations and future
avenues to resolve this tension.

We present results on the emission-line properties of 1.3<=z<=2.7 galaxies
drawn from the complete MOSFIRE Deep Evolution Field (MOSDEF) survey.
Specifically, we use observations of the emission-line diagnostic diagram of
[OIII]5007/Hb vs. [SII]6717,6731/Ha, i.e., the [SII] BPT diagram, to gain
insight into the physical properties of high-redshift star-forming regions.
High-redshift MOSDEF galaxies are offset towards lower [SII]6717,6731/Ha at
fixed [OIII]5007/Hb, relative to local galaxies from the Sloan Digital Sky
Survey (SDSS). Furthermore, at fixed [OIII]5007/Hb, local SDSS galaxies follow
a trend of decreasing [SII]6717,6731/Ha as the surface density of star
formation (Sigma_SFR) increases. We explain this trend in terms of the
decreasing fractional contribution from diffuse ionized gas (f_DIG) as
Sigma_SFR increases in galaxies, which causes galaxy-integrated line ratios to
shift towards the locus of pure HII-region emission. The z~0 relationship
between f_DIG and Sigma_SFR implies that high-redshift galaxies have lower
f_DIG values than typical local systems, given their significantly higher
typical Sigma_SFR. When an appropriate low-redshift benchmark with zero or
minimal f_DIG is used, high-redshift MOSDEF galaxies appear offset towards
higher [SII]6717,6731/Ha and/or [OIII]5007/Hb. The joint shifts of
high-redshift galaxies in the [SII] and [NII] BPT diagrams are best explained
in terms of the harder spectra ionizing their star-forming regions at fixed
nebular oxygen abundance (expected for chemically-young galaxies), as opposed
to large variations in N/O ratios or higher ionization parameters. The evolving
mixture of HII regions and DIG is an essential ingredient to our description of
the ISM over cosmic time.